Interlayer Lattice Research Articles

La3+ Doped Nickel‐Manganese Oxide as High‐Capacity Cathode for Sodium‐Ion Batteries Guided by Bayesian Optimization

In sodium‐ion batteries, the layered transition metal oxides used as cathode often experience interlayer sliding of interlayer spacing and lattice variations during charge/discharge, leading to structural damage and capacity degradation. To address this challenge, a La3+ doping strategy guided by Bayesian optimization has been employed to prepare the high‐performance O3‐NaNi0.39Mn0.50Cu0.06La0.05O2 (NMCL) cathode material. Density functional theory calculations reveal that the O 2p orbital overlaps with the t2g orbital of transition metals in NMCL, facilitating the formation of Na‐O‐La bonds and promoting the oxygen redox reaction kinetics. During the Na⁺ (de)intercalation process, NMCL exhibits significant negative lattice expansion, characterized by an increase in the c lattice parameter and exceptionally low volume expansion of 1.8% and 3.4%, respectively. Consequently, it delivers an excellent specific capacity of 243.3 mAh g‐1 over a wide voltage range of 2.0 V to 4.5 V, which can be attributed to La3+ doping that promotes oxidation of O2‐ to peroxide O2n‐ (n < 2) during charge.

La3+ Doped Nickel-Manganese Oxide as High-Capacity Cathode for Sodium-Ion Batteries Guided by Bayesian Optimization.

In sodium-ion batteries, the layered transition metal oxides used as cathode often experience interlayer sliding of interlayer spacing and lattice variations during charge/discharge, leading to structural damage and capacity degradation. To address this challenge, a La3+ doping strategy guided by Bayesian optimization has been employed to prepare the high-performance O3-NaNi0.39Mn0.50Cu0.06La0.05O2 (NMCL) cathode material. Density functional theory calculations reveal that the O2p orbital overlaps with the t2g orbital of transition metals in NMCL, facilitating the formation of Na-O-La bonds and promoting the oxygen redox reaction kinetics. During the Na+ (de)intercalation process, NMCL exhibits significant negative lattice expansion, characterized by an increase in the c lattice parameter and exceptionally low volume expansion of 1.8 % and 3.1 %, respectively. Consequently, it delivers an excellent specific capacity of 243.3 mAh g-1 over a wide voltage range of 2.0 V to 4.5 V, which can be attributed to La3+ doping that promotes oxidation of O2- to peroxide O2 n- (n<2) during charge.

In Situ Lattice-Resolution Revelation of the Origins of Unexplored Anisotropic Sodiation Kinetics and Phase Transition in the Niobium Sulfide Anode.

Layered transition metal dichalcogenides (TMDs) have exhibited huge potential as anode materials for sodium-ion batteries. Most of them usually store sodium via an intercalation-conversion mechanism, but niobium sulfide (NbS2) may be an exception. Herein, through in situ transmission electron microscopy, we carefully investigated the insertion behaviors of Na ions in NbS2 and directly visualized anisotropic sodiation kinetics. Lattice-resolution imaging coupled with density functional theory calculations reveals the preferential diffusion of Na ions within layers of NbS2, accompanied by observable interlayer lattice expansion. Impressively, the Na-inserted layers can still withstand in situ mechanical testing. Further in situ observation vertical to the a/b plane of NbS2 tracked the illusive conversion reaction, which could result from interlayer gliding or wrinkling associated with stress accumulation. In situ electron diffraction measurements ruled out the possibility of such a conversion mechanism and identified a phase transition from pristine 3R-NbS2 to 2H-NaNbS2. Therefore, the NbS2 anode stores Na ions via only the intercalation mechanism, which conceptually differs from the well-known intercalation-conversion mechanism of typical TMDs. These findings not only decipher the whole sodiation process of the NbS2 anode but also provide valuable reference for unraveling the precise sodium storage mechanism in other TMDs.

On the origin of enhanced electrochemical kinetics in guest-ions pre-intercalated layered vanadium oxides: Interlayer spacing vs lattice distortion

Cathode materials of highly stable structure and high capacity are essentially required for aqueous rechargeable zinc ion batteries (ARZIBs), where layered vanadium oxide hydration (VOH) is one of the most promising cathode materials for reversible zinc ion intercalation/deintercalation. Previous studies have shown that both the interlayer spacing and the Zn-ion storage performance can be altered by the pre-intercalation of certain guest ions between the layers of VOH. In addition to previous discussions on the increase in effective Zn2+ storage as a result of the widening in interlayer spacing after guest ion pre-intercalation, there is a variation in the exposed active O sites, which are shown to be equally important. In the metal ion pre-intercalated layered vanadium oxides, there is a delicate balance between the interlayer spacing and lattice distortion, depending on the type and amount of pre-intercalated metal ions. In particular, those pre-intercalated guest ions with high electronegativity can steadily shift towards the VO layers and break the V-O bonds leading to exposed active O sites. The high electronegativity endows the combination of guest ions with newly exposed O sites. These active O sites give rise to an inhomogeneous distribution of the charge density, leading to an improvement in electrochemical performance. The present study demonstrates the complicated interplays of more than one parameter in the guest ion pre-intercalated layered vanadium oxides, going much beyond the simple widening in inter-layer spacing.

Cross-examination of photoinitiated carrier and structural dynamics of black phosphorus at elevated fluences.

Revived attention in black phosphorus (bP) has been tremendous in the past decade. While many photoinitiated experiments have been conducted, a cross-examination of bP's photocarrier and structural dynamics is still lacking. In this article, we provide such analysis by examining time-resolved data acquired using optical transient reflectivity and reflection ultrafast electron diffraction, two complementary methods under the same experimental conditions. At elevated excitation fluences, we find that more than 90% of the photoinjected carriers are annihilated within the first picosecond (ps) and transfer their energy to phonons in a nonthermal, anisotropic fashion. Electronically, the remaining carrier density around the band edges induces a significant interaction that leads to an interlayer lattice contraction in a few ps but soon diminishes as a result of the continuing loss of carriers. Structurally, phonon-phonon scattering redistributes the energy in the lattice and results in the generation of out-of-plane coherent acoustic phonons and thermal lattice expansion. Their onset times at ∼6ps are found to be in good agreement. Later, a thermalized quasi-equilibrium state is reached following a period of about 40-50ps. Hence, we propose a picture with five temporal regimes for bP's photodynamics.

Indium defect complexes in (In x Ga1−x )2O3: a combined experimental and hybrid density functional theory study

Indium defects in small concentration (In x Ga1−x )2O3 were studied using a combination of spectroscopic and magnetic measurements on thin films varying the indium concentration, coupled with hybrid density functional theory simulations using the supercell method. X-ray diffraction spectra along with Tauc plots and density of states plots reveal a decrease (increase) in the electronic band gap (interlayer lattice spacing) due to the inclusion of indium in monoclinic Ga2O3, while room-temperature Hall measurements show an increase in n-type conductivity. Formation energy calculations reveal that the defect complex of substitutional indium at the octahedrally coordinated cation site (InGa) coupled with an indium interstitial (Ini) in the largest Ga–O cavity in the bulk (ia ), where the two impurities are a maximal distance away in the unit cell, results in the lowest formation energy across much of the electronic band gap; near the conduction band edge the single InGa defect becomes the lowest energy defect, though. These calculations help shed light on the impurity band enhanced, n-type conductivity increase due to small concentration indium doping in Ga2O3 as seen in the spectroscopic/magnetic measurements.

Effect of synthesis methods on the activity of NiO/Co3O4 as an electrode material for supercapacitor: in the light of X-ray diffraction study.

The formation of heterostructures by combining individual components (NiO and Co3O4) is a preferred approach to enhance electrochemical performance as it leads to improved charge transfer and surface reaction kinetics. In the present work, a NiO/Co3O4 composite was prepared by two methods. First, neat NiO and Co3O4 were prepared by adopting the hydrothermal method followed by the formation of the composite (i) by a hydrothermal route (NC-Hydro) and (ii) by a calcination route (NC-Cal). NC-Hydro composite shows a specific capacity of 176 C g-1 at 1 A g-1 of current density in the three-electrode system in a 2 M KOH solution as an electrolyte with 90% cyclic retention after 5000 cycles at 4 A g-1. NC-Cal shows a specific capacity of 111 C g-1 at 1 A g-1 with 75% cyclic retention. The coulombic efficiency of NC-Hydro was 86.3% while for NC-Cal it was 42.3%. The reason behind the superior electrochemical performance of NC-Hydro in comparison to NC-Cal may be the large interlayer spacing and lattice parameters of the former, which provide large space for redox reactions. The unit cell volume of the composites was more than that of the constituents. This study reveals that the composites prepared by the hydrothermal method have superior electrochemical properties in comparison to composites prepared by the calcination method.

3D Fast Sodium Transport Network of MoS2 Endowed by Coupling of Sulfur Vacancies and Sn Doping for Outstanding Sodium Storage.

A sulfur vacancy-rich, Sn-doped as well as carbon-coated MoS2 composite (Vs-SMS@C) is rationally synthesized via a simple hydrothermal method combined with ball-milling reduction, which enhances the sodium storage performance. Benefiting from the 3D fast Na+ transport network composed of the defective carbon coating, Mo─S─C bonds, enlarged interlayer spacing, S-vacancies, and lattice distortion in the composite, the Na+ storage kinetics is significantly accelerated. As expected, Vs-SMS@C releases an ultrahigh reversible capacity of 1089 mAh g-1 at 0.1 A g-1, higher than the theoretical capacity. It delivers a satisfactory capacity of 463 mAh g-1 at a high current density of 10 A g-1, which is the state-of-the-art rate capability compared to other MoS2 based sodium ion battery anodes to the knowledge. Moreover, a super long-term cycle stability is achieved by Vs-SMS@C, which keeps 91.6% of the initial capacity after 3000 cycles under the current density of 5 A g-1 in the voltage of 0.3-3.0V. The sodium storage mechanism of Vs-SMS@C is investigated by employing electrochemical methods and ex situ techniques. The synergistic effect between S-vacancies and doped-Sn is evidenced by DFT calculations. This work opens new ideas for seeking excellent metal sulfide anodes.

Unveiling the charge migration-induced surface reconstruction of Cu2MoS4 catalyst for boosted CO2 reduction into olefiant gas

Understanding the charge dynamic behavior of the catalyst is crucial to unravel the underlying mechanism for photocatalytic CO2 reduction to C2+ products. However, the structure–activity relationships are not intuitive because of the dynamic evolution of electronic and atomic structures for catalysts under the excitation state. Herein, modeling on layered Cu2MoS4 nanosheets, we for the first time observe charge migration-induced evolution of electronic and atomic structure on Cu2MoS4 catalyst during the CO2 reduction process by combining synchronous-illumination X-ray photoelectron spectroscopy (SI-XPS) with X-ray diffraction (SI-XRD). During the dissociation of CO2 molecules as well as the charge migration process under the visible light irradiation condition, the surface S atoms return back to the lattice phase, leading to the valence-state normalization of Cu, Mo and S atoms and the increases of the interlayer lattice spacing. By virtue of the above unique characteristic changes, Cu2MoS4 nanosheets exhibit excellent activity enhancement (8.7 μmol g−1 h−1) for CO2 reduction to C2H4 under the visible light irradiation in comparison with the trace amount of Cu-doped MoS2 and pure MoS2.

Structural Transformations in Ni (Oxy)Hydroxide Host Structures Under Operating Conditions for Oxygen Evolution Electrocatalysts

The incorporation of Fe into the brucite-like Ni(OH)2 host structure increases significantly the catalytic activity for the oxygen evolution reaction (OER).1 The resulting hydrotalcite-like crystalline structure is known as NiFe layered double hydroxide (LDH). However, even the most active NiFe LDH catalyst still shows a considerable overpotential for the OER.2 Understanding the nature of the catalytic active sites and the catalytic mechanism in NiFe LDHs are key challenges to develop better OER electrocatalysts, as well as the fundamental understanding of the role of the Ni-based host structure.In this contribution, atomic-scale details of the catalytic active phase will be presented, showing that NiFe LDHs are oxidized under applied anodic potentials from as-prepared α-phases to activated γ-phases.3 The interlayer and in-plane lattice parameters of the OER-active γ-phase were obtained by performing wide angle X-ray scattering (WAXS) measurements on NiFe LDH nanoplatelets during operating electrochemical conditions and were characterized by a contraction of about 8%. Operando WAXS was then performed for other selected catalysts belonging to the transition metal LDH family of materials. Structural similarities of the catalytically active phases will be highlighted. In addition, the structural transformations of β-Ni(OH)2 will be discussed, as potential host structure for novel multi-element catalysts. Also for Ni(OH)2, the γ-phase is found to be the catalytically active phase. However, the structural transformations are more complex, involving multiple limiting phases. Their structural stability is presented and the discussion supported by density functional theory calculations. References L. Trotochaud, S. L. Young, J. K. Ranney and S. W. Boettcher, Journal of the American Chemical Society, 2014, 136: 6744-6753.F. Dionigi and P. Strasser, Advanced Energy Materials, 2016, 6 1600621.F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. Ferreira de Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. Roldan Cuenya and P. Strasser, Nat Commun, 2020, 11 2522.

Lattice distortion in FCC HEAs and its effect on mechanical properties: Critical analysis and way forward

Lattice distortion is considered to be one of the four core effects in a multicomponent high-entropy alloy. However, their effect is least understood from experiment and theoretical standpoints. The present investigation revealed a unique way to understand this effect by combining experiments with density functional theory (DFT) calculations. A small amount of Al and Si were carefully added to the whole-solute matrix of Cantor alloys. The different-sized atomic species introduces a huge lattice distortion in the matrix, leading to a simultaneous improvement in yield strength (YS), ultimate tensile strength (UTS), and percent elongation. An extensive DFT simulation indicates that a lattice distortion is prominent in an Al-containing alloy, whereas Si does not induce a lattice distortion. However, Si leads to severe interlayer lattice distortion, caused by the displacement of Si, during twinning. This leads to the improvement of YS, UTS, and ductility. Lattice distortion and its variants play significant effects on the mechanical properties of high-entropy alloys (HEAs) in terms of local lattice distortion, providing an uneven energy landscape for the movement of line defects or interlayer distortion. The inherent nature of local lattice distortion in HEAs leads to the wavy or tortuous dislocation, unlike a straight dislocation in conventional alloys. The movement of the wavy type of dislocation through a distorted or defective lattice requires large stress, resulting in a pronounced effect on solid solution strengthening. This local lattice distortion also dictates the degree of the interlayer distance distortion in the vicinity of atoms, leading to an increase or decrease in stable stacking fault energy that decides the deformation mode via slip or twinning.

Interlayer Registry Dictates Interfacial 2D Material Ferroelectricity

We discover that the complex ferroelectric response of layered materials toward interlayer sliding is fully dictated by the interlayer lattice registry. Importantly, the entire sliding polarization landscape of two-dimensional (2D) layered material interfaces is fully described via a simple and intuitive geometric measure, termed the polarization registry index (PRI), that quantifies the degree of interlayer commensurability. Beyond the understanding of the fundamental origin of 2D ferroelectricity, the developed tool also provides highly efficient characterization and rationalization of existing experimental and computational evidence of 2D interfacial ferroelectricity, as well as the prediction of emergent controllable polarization in new noncentrosymmetric layered systems.

Intrinsic Catalytic Activity and Active Phase for Oxygen Evolution in Layered Double Hydroxide Electrocatalysts

The NiFe layered double hydroxides (LDHs) are among the most active electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes.1 The incorporation of Fe dramatically increases the catalytic activity of Ni hydroxides.2 However, even the most active NiFe LDH catalyst still shows a considerable overpotential for the OER. Understanding the nature of the catalytic active sites and the catalytic mechanism are key challenges to develop better OER electrocatalysts.In this contribution, atomic-scale details of the catalytic active phase will be presented, showing that NiFe LDHs are oxidized under applied anodic potentials from as-prepared α-phases to activated γ-phases.3 The interlayer and in-plane lattice parameters of the OER-active γ-phase were obtained by performing wide angle X-ray scattering (WAXS) measurements on NiFe LDH nanoplatelets during operating electrochemical conditions and were characterized by a contraction of about 8%. Operando WAXS was then performed for other selected catalysts belonging to the transition metal LDH family of materials. Structural similarities of the catalytically active phases will be highlighted.Finally, in order to derive activity-structure relationships, an approach is presented to calculate intrinsic catalytic activities, which are challenging to evaluate for this class of materials. The presented method is based on electrochemical active surface area normalization obtained by impedance spectroscopy measurements under OER.4 References F. Dionigi and P. Strasser, Advanced Energy Materials, 2016, 6 1600621. L. Trotochaud, S. L. Young, J. K. Ranney and S. W. Boettcher, Journal of the American Chemical Society, 2014, 136: 6744-6753.F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. Ferreira de Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. Roldan Cuenya and P. Strasser, Nat Commun, 2020, 11 2522.F. Dionigi, J. Zhu, Z. Zeng, T. Merzdorf, H. Sarodnik, M. Gliech, L. Pan, W.-X. , Li, J. Greeley, and P. Strasser, Angew Chem Int Edit, 60, 14446 – 14457 (2021).

Antimony‐Doped p‐Type In2Se3 for Heterophase Homojunction with High‐Performance Reconfigurable Broadband Photovoltaic Effect

AbstractHeterophase homojunction (HH) presents a new paradigm in the next‐generation broadband photodetection, light‐emitting diode, and nonvolatile memory devices. Lattice mismatch is one of the fundamental characteristics that makes the HH devices difficult to stand out. In2Se3, as a polymorphic layered semiconductor, can provide an effective approach to breaking the critical constraint on interlayer lattice mismatch. However, it is extremely important and challenging to control the crystal phase of In2Se3. In this paper, the phase‐selective growth are achieved by alloying antimony (Sb) into In2Se3 crystal structure forming In2(1−x)Sb2xSe3 crystals, where x = 0–20%. Interestingly, for x = 13%, β‐In1.74Sb0.26Se3 exhibits a p‐type semiconductor characteristic. Accordingly, an α‐In2Se3/β‐In1.74Sb0.26Se3 van der Waals p–n HH is designed and fabricated. This device not only achieves a broadband spectral photovoltaic response from the visible to near‐infrared (405–1064 nm) but also exhibits a fast photoresponse speed at about microseconds at room temperature. Moreover, the photovoltaic figure‐of‐merits can be greatly modulated by the reconfigurable built‐in potential in the p–n HH that is related to the ferroelectric polarization in α‐In2Se3. This work enables a great significance of vdW ferroelectric HHs for future photovoltaic and optoelectronic devices.

Interlayer coupling and strain localization in small-twist-angle graphene flakes

Twisted bilayer graphene (TBG) exhibits a wide range of intriguing physical properties, such as superconductivity, ferromagnetism, and superlubricity. Depending on the twist angle, periodic moiré superlattices form in twisted bilayer graphene, with inhomogeneous interlayer coupling and lattice deformation. For a small twist angle (typically < 2°), each moiré supercell contains a large number of atoms (>10,000), making it computationally expensive for first-principles and atomistic modeling. In this work, a finite element method based on a continuum model is used to simulate the inhomogeneous interlayer and intralayer deformations of twisted graphene flakes on a rigid graphene substrate. The van der Waals interactions between the graphene layers are described by a periodic potential energy function, whereas the graphene flake is treated as a continuum membrane with effective elastic properties. Our simulations show that structural relaxation and the induced strain localization are most significant in a relatively large graphene flake at small twist angles, where the strain distribution is highly localized as shear strain solitons along the boundaries between neighboring domains of commensurate AB stacking. Moreover, it is found that there exist many metastable equilibrium configurations at particular twist angles, depending on the flake size. The nonlinear mechanics of twisted bilayer graphene is thus expected to be essential for understanding the strain distributions in the moiré superlattices and the strain effects on other physical properties.

Ultrafast Carrier-Coupled Interlayer Contraction, Coherent Intralayer Motions, and Phonon Thermalization Dynamics of Black Phosphorus.

Black phosphorus (bP) exhibits highly anisotropic properties and dynamical behavior that are unique even among two-dimensional and van der Waals (vdW) layered materials. Here, we show that an interlayer lattice contraction and concerted, symmetric intralayer vibrations occur concurrently within few picoseconds following the photoinjection and relaxation of carriers, using ultrafast electron diffraction in the reflection geometry to probe the out-of-plane motions. A strong coupling between the photocarriers and bP's puckered structure, with the alignment of the electronic band structure, is at work for such directional atomic motions without a photoinduced phase transition. Three temporal regimes can be identified for the phonon thermalization dynamics where a quasi-equilibrium without anisotropy is reached in about 50 ps, followed by propagation of coherent acoustic phonons and heat diffusion into the bulk. The early time out-of-plane dynamics reported here have important implications for single- and few-layer bP and other vdW materials with strong electronic-lattice correlations.

Dynamic Tuning of Moiré Superlattice Morphology by Laser Modification.

In artificial van der Waals (vdW) layered devices, twisting the stacking angle has emerged as an effective strategy to regulate the electronic phases and optical properties of these systems. Along with the twist registry, the lattice reconstruction arising from vdW interlayer interaction has also inspired significant research interests. The control of twist angles is significantly important because the moiré periodicity determines the electron propagation length on the lattice and the interlayer electron-electron interactions. However, the moiré periodicity is hard to be modified after the device has been fabricated. In this work, we have demonstrated that the moiré periodicity can be precisely modulated with a localized laser annealing technique. This is achieved with regulating the interlayer lattice mismatch by the mismatched lattice constant, which originates from the variable density of sulfur vacancy generated during laser modification. The existence of sulfur vacancy is further verified by excitonic emission energy and lifetime in photoluminescence measurements. Furthermore, we also discover that the mismatched lattice constant has the equivalent contribution as the twist angle for determining the lattice mismatch. Theoretical modeling elaborates the moiré-wavelength-dependent energy variations at the interface and mimics the evolution of moiré morphology.

Omnidirectional exciton diffusion in quasi-2D hybrid organic-inorganic perovskites.

Exciton transport plays a central role in optoelectronic and photonic devices. In quasi-two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs), tightly bound excitons are found to diffuse within 2D layers rapidly with a non-monotonic temperature dependence. Surprisingly, the interlayer exciton diffusion is quite effective as well despite the large interlayer distance. This is in sharp contrast to electron transport, where the interlayer mobility is several orders of magnitude smaller than the intralayer one. Here, we show that the unusual exciton diffusion behaviors can be systematically modeled via the excitonic band structure arising from a long-range dipolar coupling. Coherent exciton motion is interrupted by scattering of impurities at low temperatures and of acoustic/optical phonons at high temperatures. Acoustic and optical phonons modulate the dipole-dipole distance and the dipole orientation, respectively. The ratio of intralayer and interlayer diffusion constants, Dxx/Dzz, is comparable to az/ax with az and ax being the interlayer and intralayer lattice constants of 2D HOIPs, respectively. The efficient and omnidirectional exciton diffusion suggests a great potential of 2D HOIPs in novel excitonic and polaritonic applications.

Governing Interlayer Strain in Bismuth Nanocrystals for Efficient Ammonia Electrosynthesis from Nitrate Reduction

Electrochemical ammonia (NH3) synthesis from nitrate (NO3-) reduction offers an intriguing approach for both sustainable ammonia synthesis and environmental denitrification, yet it remains hindered by a complicated reaction pathway with various intermediates. Here we present that the interlayer strain compression in bismuth (Bi) nanocrystals can contribute to both activity and selectivity improvement toward NH3 electrosynthesis from NO3- reduction. By virtue of comprehensive spectroscopic studies and theoretical calculations, we untangle that the interlayer lattice compression shortens Bi-Bi bond to broaden the 6p bandwidth for electron delocalization, promoting the chemical affinities of nitrogen intermediates. Such a manipulation facilitates NO3- activation to reduce the energy barrier for activity improvement, and also alleviates *NO2 desorption to suppress nitrite generation. As a result, a strain-compressive Bi electrocatalyst yields a maximal Faradaic efficiency of 90.6% and high generation rate of 46.5 g h-1 gcat-1 with industrially scalable partial current density up to 300 mA cm-2 for NH3 product at the optimized conditions, respectively.

Ultralow lattice thermal conductivity and high thermoelectric performance of the WS2/WTe2 van der Waals superlattice

Vertically stacked materials have received much attention due to the possibilities of effective modulation of their electronic and transport properties. In this work, we combine first-principles calculations and Boltzmann theory to investigate the thermoelectric properties of the van der Waals superlattice formed by alternately stacking the WS2 and WTe2 layers along the out-of-plane direction. Both the ab-initio molecular dynamics simulation and the phonon dispersion relations indicate that the superlattice structure is rather stable. Compared with those of the bulk WS2 and WTe2, our system exhibits much lower interlayer lattice thermal conductivity originated from the mixed covalent and van der Waals bonding. Besides, the weaker energy dispersions and higher band degeneracy along the out-of-plane direction lead to a larger power factor, which in turn gives a significantly enhanced ZT value of ∼2.4 at 800 K. Collectively, our theoretical work demonstrates the great advantage of engineering layered structure for thermoelectric applications.